Manufacture of phosphoric acid



Feb. 10, 1942. G. DL J B015 MANUFACTURE OF PHOSPHORIC ACID Filed May 22; 1959 Flames NO. l

Heuz No. 5

Gas/on DUBaQ INVENTOR ATTORNEY atented Feb. 10, 1942 2,272,402 MANUFACTURE or rnosrnomc Acm Gaston Du Bois, Clayton, Mo., assignor to Monsanto Chemical Company, a corporation of Delaware Application May 22, 1939, Serial No. 275,130

Claims.

This invention relates to the manufacture of phosphoric acid and also to apparatus therefor.

One of the objects of the present invention is the provision of a simplified and economical process for the manufacture of strong phosphoric acids. Another object is the provision of a process for the manufacture of various polyphos: phoric acids. A further object is the provision of apparatus suitable for carrying out such processes.

In known processes for the manufacture of phosphoric acid involving the combustion of elemental phosphorus and the subsequent hydration of the combustion products, it has been customary to remove a large part of the heat generated by such combustion by the evaporation of water. In accordance with these earlier processes it has been the practice either to spray the requisite quantity of water into the hot combustion gases to cool these gases, or else to pass the combustion gases directly into a large body of water or weak acid. Such prior processes produced only the relatively weak grades of phosphoric acid, by which I mean-acids having a strength of less than 85% or 95% H3PO4. Because of the relatively large quantities of water evaporated in cooling the combustion gases down to collecting temperatures, it was difiicult, if not impossible, to produce acids having strengths ranging from 95% to as high as 116% H3PO4 equivalent.

Other processes have been proposed in which phosphorus combustion gases containing an excess of water vapor are cooled to lower temperatures to produce the meta, pyro and orthophosphoric acids. Processes involving the condensation of these higher acids upon cooled metallic surfaces cannot be carried out because of the high temperatures necessary to produce such cooling of the products of combustion down to collecting temperatures, I have found that phosphoric acids of certain strengths have the property' of adhering to and covering with a continuous, viscous, flowing film, cooled metallic surfaces without corroding such surfaces. .v

. The nature of the acid film deposited upon the cooled metallic surfaces as obtained by my procmust be such as to provide fiowability and at the same time provide sufficient viscosity to impart corrosion resistance to the metal. The property of fiowability is necessary to permit a sufficient flow of heat through the metal walls and viscous flowing film of acid in order that th cooler may function efiiciently. It is evident that if the viscosity of the acid deposited upon the tubes is too great, an excessively thick film is deposited which would greatly impede the flow of heat. On the other hand if the viscosity of the acid is too low then the rate of heat flow. would be so great that the metal surface would be heated to a tempera ture at which corrosion takes place with rapid destruction of the equipment.

The nature of the acid film, that is its viscosity, is dependent upon the acid concentration and the temperature. It has been found that the acid should have a concentration of at least HsPO-i and preferably higher. For most satisfactory operation yielding an acid of high purity, that is free of corrosion products, I prefer to operate with a minimum acid strength of 104.9% H3P04 (76% P205). Acid containing less than 15 p. p. m. of nickel may be condensed upon cooled Monel tubes provided that the acid contains at least 76% P205.

The upper limit of the acid is 116% equivalent H3PO4 content which limit is fixed by the viscosity of the acid. This acid is also known as tetraphcsphoric acid, H6P40l3.

The acid collected by my process, in addition to containing .metaphosphoric acid, has an appreciable content of pyrophosphoric acid and also orthophosphoric acid. The stoichiometric composition of the acid of the limits herein specified is as follows:

equivalent H3P 0 4 Percent 104. 9 116. 0

Percent Percent Percent The viscosity of acid having an equivalent H3PO4 content of 104.9% is 200 .Saybolt seconds, while that of an acid of 116.0% H3PO4 is 7000 Saybolt seconds, both determinations having been made at 163 F.

By control of the moisture content of the air present during combustion of the phosphorus, or added prior to condensation, I may condense diess is important because the viscosity of the acid 55 rectly in fiowable form, upon the cooling surfaces, various polyphosphoric acids. Such polyphosphoric acids as are directly condensable in liquid fiowable form are the triterohexaphosphoric acid H'IP3O11; the penterohexaphosphoric acid, H5P3O1o, and the tetraphosphoric acid HsP4O13.

The triterohexaphosphoric acid has a viscosity in the neighborhood of 250 Saybolt seconds, and the penterohexaphosphoric acid a viscosity of 1500 Saybolt seconds, all measurements having been made at a temperature of 163 F.

The workability of my herein described process depends upon the discovery that phosphoric acids within the concentration and temperature limits described above exhibit practically no corrosion upon certain metallic surfaces. At the same time the viscous film of acid formed when acids within the above concentration limit are deposited upon cooled metallic surfaces has a suificient heat conductivity so as to permit of the application of this discovery to the construction of a practical cooler for cooling phosphoric acid containing gases down to the usual collecting temperatures. In the operation of my process phosphoric acid is continually deposited upon the cooled metallic surfaces from the gases in contact therewith and continually drops or flows oif therefrom. Because of the strength of the .phosphoric acid constituting the film covering the .metallic cooling surfaces, the layer of acid immediately adjacent the metal is immobile while the layers of acid farther removed from the metal surfaces are flowable due to their higher temperature; the, temperature and hence the flowability increasing with th distance the layer is removed from the metal surface. This characteristic of an acid film varying in flowability as well as temperature outwardly from the metal surface is an important feature of my process. I

By way of further explanation it may be stated that the acid deposited upon the cooled metal surfaces is pure and uncontaminated with solids as in prior art processes. The presence of a protective viscous layer of phosphoric acid upon the metal cooling surfaces is dependent solely upon the high strength of the acid thus deposited.

Metals which may offer satisfactory resistance to corrosion at the temperatures which prevail at the metal-liquid interface when strong phosphoric acid is maintained as a viscous film on the surface of the metal, according to my invention, comprise nickel-chromium steels with or without silicon, titanium, molybdenum or manganese, alloys containing a high percentage of nickel and certain casting alloys which maybe combinations of iron and silicon or complex mixtures of iron with nickel, chromium, copper molybdenum, manganese, tungsten and silicon. Although pure nickel itself is unsuited for the present purpose, its alloys, especially those which contain iron, copper or chromium, are suitable. Monel metal is particularly valuable for this purpose.

In accordance with th present invention, I

. provide cooling or heat exchange units positioned within the combustion chamber or between the combustion chamber and the absorbing system, such units being fabricated for the most part of relatively thin alloy sheets of compositions as hereinafter more particularly specified.

My process is particularly applicable to those phosphorus combustion processes wherein pure elemental phosphorus is burned with prehumidifled air. These processes are based upon the discovery that when phosphorus is burned in air containing a quantity of water vapor such that the acid contains at least three mols of water for each two mols phosphoric oxide (P205) an acid mist is produced which is easily condensed and filtered out of the gas stream and consequently a greater efficiency of collection is obtained. While such processes are capable of operation using the ordinary materials of construction, that is carbon and acidproof brick, it has been found that the acid dissolves small amounts of such construction materials, that is, alumina, silica, and iron oxides, which substances contaminate the acid produced. In cases where carbon is used as a material of construction it must be kept at a temperature below the oxidizing temperature or it must be maintained saturated with acid. In construction of, plants for the process here contemplated both of these requirements are difficult to meet, with the result that the purity of the acid produced by prior known processes is somewhat lower than-that produced by my present invention.

As a result of experiments which I have instituted on the process herein disclosed, I have discovered that corrosion by orthophosphori acid on nickel-chromium steel and high nickel alloys is favored by the low viscosity of the acid, which prevents the formation of a film of the acid adhering to the metal, with the result that acid at high temperature is in contact with the metal surface. If the viscosity of the phosphoric acid is increased by either increasing the strength of such acid, or by lowering the temperature thereof to below 180 C. and preferably below C. the corrosion may be substantially prevented because a film of appreciable thickness will be formed on the metal surface. Consequently the acid at the interface of the metal and the acid film would be of such low temperature due to the temperature drop across the film that corrosion is substantially eliminated.

In the application of this discovery to the herein described process I provide a surface of chrome-nickel alloy or other suitable metal with a film of viscous acid of such viscosity and strength as to be within the herein described limits. I maintain the viscosity and temperature of the film of acid by the application of a cooling fluid to the chrome-nickel alloy in a manner to be hereinafter more fully described.

Referring to Figure 1 of the drawing, I have shown by numeral in a brick-lined combustion chamber provided with an inlet pipe II by. which molten yellow phosphorus is supplied to said chamber. By means of pipe I2, I introduce a quantity of previously humidified air from a source not shown, the quantity of air thus supplied being so proportioned with respect to the phosphorus supplied by pipe II, that a substantial excess of oxygen over that required to burn such phosphorus is present in chamber 10. The amount of water carried by said humidified air is such that an acid having a strength of from 104.9% to 116% H3PO4 equivalent is produced.

Combustion chamber It may contain baflle walls i 3, the function of which is to thoroughly mix the phosphorus, air and moisture undergoing reaction in said chamber. The products of the reaction taking place in said chamber In pass outwardly through duct H, which, in accordance with my invention, is made of nickel-chromium alloy steel sheet or drawn tube or of a high nickel alloy in similar form and provided with cooling water sprayed upon the exterior thereof, as by pipe 15 supplying sprays It. The gases pass from duct or tube It into chamber ll which is provided witha filter medium I9 consisting of finely divided coke supported upon a grid [9. The gases pass upwardly through the filter bed l8 and out of the chamber I! by means of stack 20.

In accordance with my invention chamber I1 is constructed of nickel chromium steel sheet in a similar manner to duct l4, and is provided with spray cooling means 2i, said means consisting in this instance of a perforated pipe supplying cooling water to the exterior of the steel sheets or plates comprising filter chamber H.

The cooling water from sprays l6 and 2| is collected by means of pipe 22 and then pumped back to pipes I and 2| by pumps not shown in the drawing.

The strong acid collected in conduit I4 and filter chamber I1 is removed from the latter by means of outlet 23 and conveyed to suitable storage tanks. v

A further embodiment of my invention is illustrated by Figure 2, wherein parts numbered similarly to thosein Figure 1 have the same function as above described.

In the operation of the apparatus shown in Figure 2, the combustion gases pass out of combustion chamber I0 by means of duct I4 and enter bricklined chamber25. This chamber is provided with a multitudeof pipes or pipe coils 26 connected to headers 21, which in 'turn are connected to a main header 28. The pipes 21 are maintained full of water, thermal circulation being maintained with the water in drum 28 in the usual manner. The temperature of the water in coils 26 is maintained below 180 C. and preferably below 140 C. Said coils are constructed of the metals or alloys hereinafter enumerated and during operation are covered with a fiowable film of viscous acid in accordance with my herein described invention. Phosphoric acid of high strength and of high viscosity is condensed upon coils 26, said coils being below the dew point of the vapors and the fiowable film of acid serves to protect the material of said coils against corrosion.

The gases and vapors which are thus cooled by contact with said coils 26 pass into duct'29 and thence into chamber I'I provided with filter material l8 supported upon grid l9. Acid condensed in chamber 25 passes into chamber l1 and together with acid collected in said chamber, flows out through port 23 into suitable storage tanks.

As materials suitable for the construction of duct I4, chamber l1 and tubes 26, shown in the above apparatus in the described process, I recommend the following:

Nickel-chromium steel containing 10-30% chromium, 8-35% nickel and small amounts of molybdenum or titanium, the balance being substantially all iron. Such mafierial is known as stabilized stainless steel.

Various alloys in which nickel and copper such as Monel metal are the predominating metals.

These alloys may also contain appreciable'quantities of iron, manganese, silicon and aluminum.

Casting alloys which contain nickel, molybdenum, chromium and iron as their main constituents with smaller quantities of copper, manganese, tungsten and silicon and casting alloys consisting mainly of iron and silicon.

Having now particularly described my invention and the manner in which it can be used, I-

desire that it not be limited except as required by the prior art or as particularly pointed out in the appended claims.

This application is a continuation in part of my earlier filed application, Serial No. 160,398, filed August 23, 1937.-

. -What I claim is:

1. A process for producing pure phosphoric acid of strength within the range of 104.9% to 116% equivalent H3PO4 content, comprising burning pure yellow phosphorus in air, adjusting the water vapor content of the hot combustion gases so that the oxidation products are hydrated to the extent of containing phosphoric acid having a concentration of between 104.9 and 116% equivalent H3PO4 content, contacting said hydrated oxidation products with a resistant metallic surface, said surface being maintained constantly at a temperature below 0., con- -densing upon said cooled metallic surface a flowable film of phosphoric acid having a concentration between 104.9% and 116% equivalent H3PO4, said film being substantially immobile adjacent said metallic surface and increasing in flowability with distance outwardly from said metallic surface, and recovering acid from said fiowable film.

2. A process for producing pure phosphoric acid of strength within the range of 104.9% and 116% equivalent H3PO4 content, comprising burning pure phosphorus in air, adjusting the water-vap0rcontent of the hot combustion gases so that the oxidation products are hydrated to the extent of containing phosphoric acid having a concentration within said limits, contacting said hydrated oxidation products with the surface of a water cooled Monel tube, said cooling water being maintained at a temperature below 180 C., condensing upon said cooled Monel tube a fiowable film of phosphoric acid within said limits, said film being substantially immobile adjacent said Monel surface and increasing in flowvability with distance outwardly from said Monel surface, and maintaining said acid film by condensation from the hydrated combustion prodnets.

3. A process for producing pil e phosphoric acid of strength within the range of 104.9% to 116% equivalent H3PO4 content, comprising burning pure yellow phosphorus in air, adjusting the water vapor content of the'resulting hot combustion gases so as to contain an equivalent phosphoric acid of 104.9% to 116% H3PO4, contacting said hot hydrated combustion gases with the exterior surface of an internally Water-cooled Monel tube, said cooling water being maintained below 180 C., condensing upon said cooled Monel tube a flowing film of acid of strength within said limits, saidfilm of acid being substantially immobile adiacent said Monel surface-and increasing in fiowability with distance outwardly from said Monel surface, and recovering" acid condensed upon said Monel surface.

4. A process for producing pure phosphoric acid of strength within the range of 104.9% to 116% equivalent H3PO4 content, comprising burning pure yellow phosphorus in air, adjusting the water content of the hot combustion products so that the contained phosphorus pentoxide is' hydrated to a degree corresponding to an acid of from 104.9% to 116% equivalent H3PO4, contacting said hydrated combustion products with one side of a resistant cooled metallic sheet, said metallic sheet being maintained below 180 C. by contact with cooling water maintained at a temperature below 180 C. upon the other side thereof, condensing upon said metallic sheet a film of phosphoric acid of strength within said limits, to said cooling water, and recovering acid or said said film being substantially immobile where it strength from said film.

contacts said metallic sheet and said film in 5. The process defined in claim 4 in which the creasing in'fiowability and temperature with distemperature of the cooling water in contact with tance outwardly from said metallic sheet, and 5 saidmetallic sheet is maintained at a temperaconducting heat of condensation of said'acid ture below 140 C. through said film and through said metallic sheet GASTON DU BOIS. 

